Grid-mapped freeze-fracture analysis of gap junctions in gray and white matter of adult rat central nervous system, with evidence for a "panglial syncytium" that is not coupled to neurons.

In white matter regions of the brain and spinal cord of adult mammals, gap junctions previously were observed linking astrocytes to astrocytes, as well as to oligodendrocytes and ependymacytes. The resulting "functional syncytium" was proposed to modulate the ion fluxes that occur during electrical activity of the associated axons. Gap junctions also have been reported linking neurons with glia, and functional neuronal-glial coupling has been postulated. To investigate the glial syncytium and the neuron-to-glial coupling hypotheses, we used "grid-mapped freeze fracture," conventional thin-section electron microscopy, and light microscope immunocytochemistry to examine and characterize neurons and glia in gray and white matter of adult rat brain and spinal cord. We have obtained quantitative evidence for the abundance and widespread distribution of gap junctions interlinking the three primary types of macroglia throughout both gray and white matter of the mammalian central nervous system (CNS), thereby extending the concept to that of a functional panglial syncytium. In contrast to previous reports, we show that of more than 400 gap junctions in which both participating cells were identified, none were between neurons and glia. Thus, neuronal coupling and glial coupling involved separate and distinct pathways. Finally, putative water channels (i.e., "square arrays") were confirmed to be abundant and in close association with gap junctions in astrocytes and ependymacytes. Because the astrocyte "intermediaries" extend cytoplasmic conduits throughout gray and white matter of brain and spinal cord, from the ependymal layer to the pia-glial limitans, and from oligodendrocytes surrounding axons to astrocyte endfeet surrounding capillaries, the proposed panglial syncytium, with its abundance of water channels and intercellular ion channels, is optimally positioned and equipped to modulate water and ion fluxes across broad regions of the CNS.